1. Magnetic Particle Testing
This method is suitable for the detection of surface and near surface discontinuities in magnetic material, mainly ferrite steel and iron. Magnetic particle Testing (MPT) is a nondestructive testing method used for defect detection. MPT is fast and relatively easy to apply, and material surface preparation is not as critical as it is for some other NDT methods.

2. Basic Principle of MPT S N N S S N
Magnetic Flux Leakage
Crack
Magnetic Flux Line S N N S S N
Fig.1: Magnetized Metal with no crack
Fig.2: Magnetized Metal with crack
In the first figure the magnetized metal has no crack and there only two poles that is north pole and south pole. And in second figure the magnetized metal has a crack and at the crack point there creates another north and south pole for the magnetic flux leakage.

3. Magnetic Particle Testing Process
The first step in a magnetic particle testing is to magnetize the
test component by a MPT equipment. If there any defects on the surface or near to the surface are present, the defects will create a leakage field.
Then finely milled iron particles coated with a dye pigment are applied to the specimen. These particles are attracted to magnetic flux leakage fields and will cluster to form an indication directly over the defects. This indication can be visually detected under proper lighting conditions. 3

4. Magnetic Particle Testing in Superheater Pipe Welding
First the welding joint is magnetized by MPT equipment. Then finely milled iron particles are applied to the magnetized weld joint.

5. Magnetic Particle Testing in Gas Pipe Welding
Iron particles make a
cluster at the welding
joint for magnetic
flux leakage because
of welding defects.
Cluster

6. Advantages Magnetic Particle Testing
It does not need very stringent pre-cleaning operation.
It is the best method for the detection of surface and near to the surface cracks in ferromagnetic materials.
Fast and relatively simple NDT method.
Generally inexpensive.
Will work through thin coating.
Highly portable NDT method.
It is quicker.

7. Limitations of Magnetic Particle Testing
Material must be ferromagnetic.
Orientation and strength of magnetic field is critical.
Detects surface and near-to-surface discontinuities only.
Large currents sometimes require. .